The invention relates to a process for the agglomeration of solids, in particular of finely divided solids, suspended in a liquid.
Agglomeration is a well known process in separating solids from a carrier liquid and/or solid contaminating material. A typical example is the agglomeration of coal fines for facilitating the separation of said coal particles from water, used as a carrier liquid during the transportation by pipeline of the coal fines. Other examples are the agglomeration of coal fines for separating the coal from gangue, upgrading of coal fines for use in blast furnaces and enrichment of ores.
In general, agglomeration is carried out by bringing the solids suspended in a liquid into contact under conditions of turbulent flow with a binding agent. The binding agent is so chosen that it is capable of wetting the surface of the solids. The binding agent binds the solids together to form agglomerates, which can be easily separated from the liquid by mechanical means, such as a sieve. In case finely divided solids are to be separated from a liquid in which also solid-contaminating material is suspended, the binding agent is so chosen that it wets the surface of the solids to be separated preferentially over that of the solid contaminating material. In this manner only agglomerates of the concerned solids are formed, which can easily be separated from the remaining suspension of the solid contaminating material.
In the case of coal to be separated from gangue the solids are suspended in a finely divided form in water. This suspension is brought into contact under conditions of tubulent flow with an oily material such as fuel oil, bitumen, naptha, coal tar and the like. Such materials expel the water from the coal particles and not from the gangue. Depending on concentrations, binding agent and flow conditions, various types of agglomerates may be obtained, ranging from loosely fluffy material to hard pellets.
In the last years there is a tendency to carry out processes for the separation of solids from contaminating material on an ever increasing scale. Increasing amounts of domestic and industrial effluents, containing waste material become free in technologically advanced societies. These effluents, which tend to pollute water courses, land and the atmosphere, form a major hazard in an advanced society. For that reason effluent treatment processes to separate the waste material from effluents need to cope with these large amounts of effluent in order to produce clean potable water supplies to satisfy domestic and industrial requirements. Further, coal gets an ever increasing importance as an energy source in the nearest future. In the mining industry large amounts of coal fines contaminated with gangue and very often also with other contaminations, such as clay, are obtained. These coal fines should be separated from the contaminations and bound together to larger coal particles which are easy to handle.
Prior patents have suggested various uses of stepwise treatments for agglomerating slurries of finely divided solids. For example, British Pat. No. 1,388,371 suggests flowing a stream of the suspended fines and a binding agent through a particle-agglomerating zone of turbulent flow, screening-out the agglomerates having diameters exceeding 0.25 millimeters, grinding portions of the screened-out agglomerates to sizes of from 0.01 to 0.5 millimeters and recycling the ground particles to serve as nuclei for the agglomeration process. Such a procedure tends to promote particle growth by the coalescence machanism and is particularly useful for decreasing the amount of contaminating material being included in the agglomerates. British Pat. No. 1,390,827 suggests agglomerating such liquid-suspended fines with a binding agent in a zone of turbulent flow, separating the agglomerates, and then re-treating the agglomerates in contact with additional binding agent and a liquid containing less contaminating materials than the first-used liquid. This also tends to reduce the contamination of the agglomerates.
But, in order to cope with the ever increasing amounts of solids to be separated, it is important that the agglomeration processes should be less time consuming and should produce sufficiently large agglomerates for further handling, such as separation on a screen. A particular object of the present invention is to provide a relatively rapid process for agglomerating fine solids into relatively large and substantially uniform agglomerates.